Re: kvm_regs vs kvm_sregs?

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Hollis Blanchard wrote:
> What is the distinction between kvm_regs and kvm_sregs? As far as I can
> see, kvm_regs is only used when emulating IO, emulating MMIO, and
> emulating CPUID, where guest GPRs are directly modified. kvm_sregs is
> only used for full CPU state save (for later restore).
>   

After the userspace interface changes, neither of them are used except 
for migration, savevm/loadvm, and debugging. Theoretically they could be 
unified, practically SET_SREGS is a dangerous operation on Intel cpus 
due to kvm's very imperfect real mode support.

> When the kernel had to use copy_to_user() to transfer state to userland,
> I can see that this split resulted in less memory copying. However, now
> that userland can directly map register state without a copy, why not
> combine the two structures?
>   

We don't map the register state as that would require us to sync it on 
every exit.

> I guess it takes longer to copy state out of the VMCS into kvm_sregs, so
> why bother if userspace isn't going to use it?
>   

When it's needed, it's really needed.  Or did I misunderstand the question?

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


-------------------------------------------------------------------------
Take Surveys. Earn Cash. Influence the Future of IT
Join SourceForge.net's Techsay panel and you'll get the chance to share your
opinions on IT & business topics through brief surveys-and earn cash
http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV